The developmental patterns of substance P (SP) neurons in the central nervous system of rats has been examined by determining the SP concentration in various brain regions of prenatal and neonatal animals. During the week prior to birth, there is a rapid rise in substance P content in all the gross brain regions. There is a precipitous reduction in brain substance P on the day of birth, but within 3 days a regional specific pulse resumes. The possible interaction of SP neuronal components with the endocrine system was also investigated in pregnant, hypophysectomized and sphlanchnicectomized rats. Bilateral sphanchnicectomy results in a significant increase in brain substance P content. Significant alteration in SP content was observed by neuroendocrine manipulation. Finally, in an effort to further probe the molecular mechanism of action for substance P an anti-idiotypic antibody was raised against anti-substance P immunoglobulin.